Bangladeshi police say they have arrested two owners of garment factories based at the eight-storey building that collapsed outside the capital, as the death toll rose to at least 324.

"We've arrested Bazlus Samad, the chairman of New Wave Buttons and New Wave Style factories, and Mahmudur Rahaman Tapash, a managing director of one of these plants, after midnight," Dhaka's deputy police chief, Shyaml Mukherjee, said on Saturday.

Bangladeshi building collapse

Bangladeshis watch the rescue operations at the site of a building that collapsed Wednesday in Savar, near Dhaka, Bangladesh. The death toll reached at least 194 people as rescuers continued to search for injured and missing, after a huge section of an eight-story building that housed several garment factories splintered into a pile of concrete. Photo: AP Photo

Police have filed a case against them for "death due to negligence", he said, after the prime minister said the owners forced their staff to return to work despite cracks having appeared in the building a day earlier.

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Survivors said the building developed visible cracks on Tuesday evening, but factory bosses had demanded staff return to the production lines despite a police evacuation order.

A manager at the New Wave Styles company, one of the five manufacturers in the building, said the owner had consulted an engineer but then ignored his warnings.

"Those who are involved, especially the owner who forced the workers to work there, will be punished," Prime Minister Sheikh Hasina told MPs.

"Wherever he is, he will be found and brought to justice."

The death toll in the disaster rose to 324 early on Saturday after rescuers removed more bodies - most in a state of decay - from the wreckage, Mukherjee said.

He said more workers were pulled out alive as the desperate hunt for survivors continued through the night.

Late on Friday, Bangladeshi fire service deputy director Sheikh Mizanur Rahman said about 50 people had been found alive at several places on the third floor after digging tunnels.

"We hope we can rescue them by tomorrow morning," he said.

The discovery of more survivors brought new hope to the thousands of desperate relatives huddled at the disaster site but an intense stench of decomposition suggested many bodies remain trapped.

More than 2300 people had been rescued alive since the collapse but many are severely injured, the army said.

Exhausted rescue teams of soldiers, firemen and volunteers using concrete-cutters and drilling machines were racing against time in searing heat to find more survivors from the country's worst industrial disaster.